A particularly important area of application is where the lifting anchor is placed in the edge of an end or side of a thin concrete panel and where the panel is to be lifted in a direction normal to the axis of the anchor with a shear load applied to the anchor. A common application of this type exists in the construction of tilt-up buildings. In this type of construction thin wall panels are cast horizontally at ground level and raised into the vertical position by tilting the horizontal panel about one (lower) edge by lifting with anchors set in the opposite (upper) edge until it is in the vertical position. Then it is lifted into its final position to form a wall element.
The invention is not, however, limited to tilt-up operations and can be used in any application where an anchor is set either in an edge, or close to an edge, and where a force is to be applied in a direction towards the edge. Here there is a risk of failure of the concrete in the region of the edge as a result of the shear forces generated by the application of the force to the anchor. In such applications there is only a small distance between the anchor axis and the surface of the concrete panel in the direction of the applied force. Therefore there is often an insufficient volume of concrete to resist the applied load without failure.
Lifting anchors now in widespread use comprise a bar which at one end has a hole through which is threaded a reinforcement member. Alternatively, the one end of the bar has an enlarged foot. The reinforcement member or foot provides an anchorage to the concrete inside the panel. The other end of the bar provides a connection to the lifting device. Such anchors are commonly forged from steel. The means of connection can either be an enlarged head or a hole. Such lifting anchors and the systems by which they are lifted are described in U.S. Pat. No. 3,499,676 (1970), U.S. Pat. No 3,883,170 (1975) and U.S. Pat. No. 4,173,367 (1979) amongst others.
It is desirable to ensure that the head of the anchor to which the lifting device is attached does not protect from the concrete surface and a recess in the concrete is formed around the anchor for that purpose. The recess is commonly formed using a recess former which typically comprises a solid hemisphere which has a hole in the pole of the hemisphere into which the anchor head is placed and retained with a rubber grommet. The base of the hemisphere is attached to the interior of the mould wall. Such attachment is typically achieved by a number of holes passing through the hemisphere and back through which are passed bolts or nails. These enable the recess former to be directly nailed or bolted onto the face of the mould. In some applications the recess former is bolted onto the form or, in the case of steel moulds, a steel former can be directly welded to the form.
Another type of recess former in common use comprises two quarter-spherical parts inter-connected by a hinge (as disclosed in U.S. Pat. No. 4,296,909) or otherwise fastened together. These types of recess formers are moulded from steel, rubber or plastics and have internal structures designed to tightly retain the head of the anchor when the recess is closed about the shaft of the anchor. This provides positive support for the assembly fixed to the mould wall. This type of recess is commonly bolted to the mould wall using a centrally locating bolt passing into the flat portion of the recess former.
Another type of recess former comprises a steel hemisphere which is bored with a central tapered hole thereby forming a tapered ring element into which are fitted two or more identical, externally tapered, collets with an internal form designed to accept the head of the lifting anchor. Provision is made to draw the collets together and fix these to the mould wall once the assembly has been put together.
Yet another type of recess former is moulded from two identical quarter spheres of thin plastics material each of which has projections and slots moulded into the internal cavity to tightly retain the head of the anchor once the two halves are closed together around the anchor shaft. This type of recess former is commonly provided with a means of clipping the two halves together using pegs and holes or other fastening means moulded into the plastics. This type of recess former once fastened around the anchor is substantially secure and does not normally require fixing to a mould surface. Projections and clips can be moulded into the external face to retain a frame (e.g. of wire or a steel plate) by which the assembly can be supported during the moulding of the concrete.
In some embodiments of this type of recess former, the two halves of the recess former are firmly held together with an encircling wire frame which is tightly fitted around the recess former at a position designed to provide reinforcement against the shear forces generated when the anchor is placed in the edge of a panel. Supporting legs of wire are commonly fitted to the frame. They project into the concrete and serve as support legs for positioning the anchor when placed in the top of a horizontally cast panel or additional reinforcement and anchorage when the anchor is placed in the edge of a panel. Such assemblies are commonly placed into the edges of concrete panels by nailing onto timber formwork through the plastics or by tying the steel wire frame into the panel reinforcement. One such recess with special closure clips has been disclosed in Australian Patent No. AU-B-12822/88.
Each of these various types of recess formers can be provided with an exterior shape which is substantially hemispherical. In some cases the recess former is substantially a truncated hemisphere which produces a recess in the concrete which is slot-like when the recess former is removed to expose the anchor after the concrete has cured and hardened.
Lifting anchors can be effectively embedded in the face of precast concrete elements such as panels or beams and the longitudinal forces applied during lifting have not presented significant problems. However, when a shear load is applied i.e. force is applied at an angle to the axis of the lifting anchor, particularly where the anchor is near the edge of a precast panel, cracking of the concrete adjacent to the anchor has occurred. While this may not represent a problem structurally, the cracking is unsightly and unacceptable architecturally. Consequently costly patching and repair operations have been required.
The most common method which has been used to reduce the failures resulting from these shear forces has been to attempt to distribute these forces into the concrete by conducting the forces to an area away from the critical zone using reinforcement bars (shear bars). Attempts to provide an effective means of using anchors for receiving shear forces have included the disclosure of U.S. Pat. No. 4,087,947. Here extra reinforcing bars curve over the lifting anchor. The shear bar described in that patent is not effective for preventing damage to the concrete as a result of shear forces being transmitted by the lifting device to the inside surface of the recess in the concrete.
In order to reduce the damage it is known to use additional shear bars wrapped tightly around the recess former at a position in close approximation to the area of contact of the lifting device. Some recess formers of the types previously described and which have provision for fitting a circumscribing steel shear bar or wire have been used with some success (in this connection see Australian Patent No. AU-B-12822/88).
The shear bar is designed to accept the shear load which would otherwise be applied directly to the concrete and to transfer this load away from the concrete above the anchor to a position below the anchor. Such shear bars extend to a position below the level of the recess and have lateral extension pieces which provide anchorage. When the lifting device is forced against the shear bar by the applied shear load, the vertical section of the bar transmits the load to the anchored extension pieces in tension.
Another type of shear reinforcement uses a steel plate which has a centrally formed hole designed to accept the external form of the recess former (commonly used with truncated hemispherical formers which have two flat sides). This plate is turned inwardly towards the interior of the concrete at each end to provide a means of anchorage to the concrete. The plate is fitted around a recess which has provision for that purpose moulded into its external surface to ensure that it is positioned spatially at the optimum position to accept the shear load imparted by the lifting device.
The use of shear bars and plates and other similar variants of this type have been extensively tested in practical applications and in the laboratory and have been found to provide only a partial solution to the problem.
A different approach is that disclosed in U.S. Pat. No. 4,173,856. This patent describes a specially shaped anchor which prevents the lifting device making contact with the concrete element and which has special longitudinal extensions on the sides of the anchor which transmit the total applied shear force to the anchor. The anchor incorporates apertures for engagement of reinforcement rods through which the shear force is transmitted to the panel below the axis of the anchor. This solution has been found to be effective in most cases as failure has mainly occurred in very thin panels.
A disadvantage of the existing methods of transferring the shear loads into the concrete by using steel reinforcement bars has been that the bars have been directly embedded into the concrete and no account has been taken of the different physical properties of concrete and steel. The elastic modulus of cured concrete is very much greater than that of the steel used to provide the shear reinforcement.
When the force is applied to the steel it cannot be transferred by the steel until the steel extends elastically. The steel is restrained by the surrounding concrete. Extension of the steel is only possible if the surrounding concrete has an elastic modulus less than the steel. If the concrete is uncured its modulus can be less than that of the steel and it can permit the movement of the steel and the forces will be transferred by the steel. In most cases however the concrete has already cured and no transfer of the forces can take place until the concrete cracks and allows the steel to extend until the elastic force of the steel is equal to the applied force. This is the principle on which the theory of steel reinforcement of concrete is based.
A further disadvantage of existing methods using shear bars is that failure of the concrete panel can occur by bursting from the edge where the anchorage extension of the shear bar extends in the same plane as the edge of the panel (normal to the anchor axis). The applied force imparts a rotational force to the anchorage. Cracks opened by this force can initiate a failure crack which propagates in the direction of lift. Such cracking can occur at applied loads which are less than the loads required to cause the panel to fail in the direction of lift.
There is therefore a need for a device which ensures that the shear forces can be transferred by the reinforcement without prior concrete cracking. In many handling operations there are load reversals especially when manipulating a concrete member by rotation through 180 degrees. The device is therefore preferably capable of reinforcement in both directions.